Thermochromic encapsulated dyes undergo a color change over a specific temperature range. By way of example, a dye may change from a particular color at low temperature to colorless at a high temperature, such as red at 85° F. and colorless at above 90° F. The color change temperature is controllable, such that the color change can take place at different temperatures. In one example, the color change may occur at a temperature just below a person's external body temperature so that a color change occurs in response to a human touch. For those skilled in the art of thermochromic microcapsule synthesis, the precise control of the temperatures at which color changes occurs is easily achievable. For example, the ideal temperature of color change for cool beverages may range from 0° C. to 15° C., while the ideal temperature change for a warm or hot beverage may lie between 40° C. to 65° C.
Thermochromic systems consist of three main components: an electron donating chromophore, an electron-accepting color developer and a non-polar solvent that facilitates color change over a specified temperature range. The properties of thermochromic systems have been exploited for more than 35 years. One technique used to produce the thermochromic encapsulated dye is to combine water, dye, and oil with melamine formaldehyde resin and agitate to create a very fine emulsification. Interfacial tensions are such that the oil and dye end up on the inside of a melamine formaldehyde capsule distributed in primarily the water phase. The melamine formaldehyde substance, while very hard and resistant to breakdown at high temperature, is permeable. Though there has been significant improvement in microencapsulation technology, thermochromic systems still have inherent chemical instability in polar solvent-based systems. For this reason, microencapsulated thermochromic pigments have found limited applicability in solvent-based systems. For example, U.S. Pat. No. 6,139,779, describes how low molecular weight solvents (generally less than 100 g/mol) have been shown to permeate the relatively thin microcapsule wall and destroy the thermochromic system. A variety of thermochromic inks may be purchased on commercial order, for example, from Chromatic Technologies, Inc. of Colorado Springs, Colo.
U.S. Pat. Nos. 4,421,560 and 4,425,161 entitled “Thermochromic Materials” both state that thermochromic inks can be made with “conventional additives used to improve conventional printing inks.” Nonetheless, there are concerns over what additives may be added to these inks due to materials incompatibility issues.
Thermochromic dye is often sold in a slurry of pigment, formed of encapsulated dye in a water base. It happens that the pH of this slurry is most often neutral in a range from 6.5 to 7.5. When thermochromic dye is added to a formulation that has a pH outside this range, the color change properties are often lost. This can be an irreversible effect and, therefore, it is important to adjust the pH prior to adding the thermochromic dye.
Several types of ingredients are traditionally added to ink formulations. The combination of all the ingredients in an ink, other than the pigment, is called the vehicle. The vehicle carries the pigment to the substrate and binds the pigment to the substrate. The correct combination of vehicle ingredients will result in the wetting of an ink. This wetting means that the vehicle forms an absorbed film around the pigment particles. The main ingredient in an ink is the binder. This may be a resin, lacquer or varnish, or some other polymer. The binder characteristics vary depending on the type of printing that is being done and the desired final product. The second main ingredient is the colorant itself, for example, as described above. The remaining ingredients are added to enhance the color and printing characteristics of the binder and the colorant. These remaining ingredients may include reducers (solvents), waxes, surfactant, thickeners, driers, and/or UV inhibitors.
Thermochromic inks have been used successfully as indicators of a preferred usage temperature and as a brand differentiator. Specifically, thermochromic inks have been used as cold indicators on aluminum cans, via metal decorating inks, to communicate optimum consumption temperature to the consumer. This interactivity through thermochromic color change so far does not extend to coating on can ends, tabs, caps and closures. To date, no such coatings are commercially available. In part, this is due to significant mechanical forces that are applied to pre-coated coil stock to form can ends, tabs, caps and other closures. Because of the stress and sheer during the tooling process the coating must be flexible and resistant to cracking, flaking, and other damage. In addition, the coating must be sufficiently chemically resistant to be unaffected by pasteurization or other processes. In order to meet the above requirements, the reversible thermochromic coating described herein must contain a thermochromic pigment, a resin, and a commercially available coating commonly used for can and coil coatings. In order to engineer additional coating properties, for example, chemical resistance or flexibility, components such as a curing agent, an accelerator or catalyst to enhance curing, or wax, may be added. Furthermore, thermochromic microcapsule wetting agents may be incorporated to aid pigment dispersion, and one or more solvents may be selected.
Plain lids of the type used in beverage cans are stamped from a coil of aluminum, typically alloy 5182-H48, and transferred to another press that converts the stamped materials into easy-open ends. The conversion press forms an integral rivet button in the lid and scores the opening, while concurrently forming the tabs in another die from a separate strip of aluminum. The tab is pushed over the button, which is then flattened to form the rivet that attaches the tab to the lid. The top rim of the can is trimmed and pressed inward or “necked” to form a taper conical where the can will later be filled, and the lid (usually made of an aluminum alloy with magnesium) attached. The lid components, especially the tabs, may be coated with various coatings for use in can ends, tabs, caps or closures before they are subjected to such manufacturing processes as riveting.
Beverage cans are usually filled before the top is crimped in place. The filling and sealing operations are fast and precise. The filling head centers over the can and discharges the beverage to flow down the sides of the can. The lid is placed on the can then crimped in two operations. A seaming head engages the lid from above, while a seaming roller to the side curls the edge of the lid around the edge of the can body. The head and roller spin the can in a complete circle to seal all the way around. A pressure roller next drives the two edges together under pressure to make a gas-tight seal. Filled cans usually have pressurized gas inside, which stiffens the filled cans for subsequent handling.
United States Patent Application publication number 2003/01274515 A1 describes the use of thermochromic inks to apply printable images to metal lids and caps. United States Patent Application publication number 2011/0226636 A1 describes the use of thermochromic inks as applied in multiple ways to aluminum can ends, the displaceable tear panel and the non-detachable tab. While these disclosures describe the application of thermochromic inks, they do not teach practical means of achieving the claims described. Conventional thermochromic inks are generally unsuited for the manufacturing stresses involved in making the can ends and tabs which, practically speaking, are made from aluminum rolls, commonly known as coil stock, that must be coated prior to the machining operations that form the can ends and tabs. Because of the durability and chemical stability of coatings, they are commonly applied to aluminum and metal cans where direct or indirect food contact may occur. The art does not, however, provide detailed formulations for reversible thermochromic resin systems, for use in can ends, tabs, caps or closures.
It is problematic that existing thermochromic coatings fail to withstand the stresses of these manufacturing operations which may, for example, be excessively thin or scratch the coatings or crush the micro capsules forming the thermochromic pigment.
Prior attempts at printing thermochromic inks on the ends of cans have failed because the inks cured too slowly. Prior attempts to add acid catalysts to quicken the rate of cure have failed because the acid catalysts permanently activate the thermochromic pigment in the inks.